shader: Add bgra-rgba swizzling on FS outputs only when there are actual color outputs

In an FS that only writes depth (e.g), num_outputs > 0, but we must not emit fs_out swizzling code, because there are no color outputs, so check for this.

To help debugging issues like these with shaders, also keep the TGSI text when compiling in debug mode and dump it too when the GLSL is dumped.